The present invention relates to a brushless motor control system which controls currents supplied to multi-phase coils of a brushless DC motor to effectively control the motor.
There has been proposed a brussless motor control system which is employed in a power assist system. This brushless motor control system is arranged to estimate a phase angle of a brushless DC motor by executing a phase-angle estimating method using inexpensive Hall-effect sensors. The power assist system employing the brushless motor control system is generally arranged to assist a manipulating force using a motor torque and is applied to a power steering system or a conveying system. The power assist system comprises load detecting means for detecting a load generated by an operator and controls a command current to the motor so as to bring the detected load closer to a target value. More specifically, the assisting force of the motor is increased by increasing the command current as the load increases. Inversely, the assisting force is decreased by decreasing the command current as the load decreases.
However, this proposed brushless motor control system has the following problem.
In the power assist system employing a motor torque as an assisting force, when an output torque is lowered due to the phase angle shift, the assisting force by the motor is lowered. As a result, the load (torque reaction) increases and the detection value of the load detection means also increases. Therefore, current control means of the power assist system increases the command current. As shown in FIGS. 4A through 4D, a phase angle shift is generated during a period B. The assisting force is decreased according to the lowering of the motor torque due to the phase angle shift, and therefore the torque signal outputted from the torque sensor increases and the command current also increases. Thereafter, the position signal Ps12 is obtained at the start of a period C, and an estimated phase angle is corrected to the actual phase angle obtained in response to the input of the position signal Ps12. Therefore, the effective current supply to the motor is achieved.
However, in this corrected state, the command current, which is generated so as to compensate the shortage of the assisting force, becomes excessive. That is, a torque signal Vt is lowered by the excess of the assisting force. This also lowers the command current inversely. Accordingly, in this related art, when the phase angle shift is generated by the advance of the estimated phase angle relative to the actual phase angle and when the position signal is obtained, overcurrent is outputted by the execution of the correcting processing for correcting the estimated phase angle to the actual phase angle. This generates a hunting of the torque and therefore an operator has a strange feeling to this proceeding in this control operation.
It is therefore an object of the present invention to provide an improved brushless motor control system which improves a controllability of a motor so as to suppress a hunting which is caused by overcurrent due to a phase angle shift, while estimating a phase angle of the motor by measuring a change and a time of a position signal using inexpensive Hall-effect sensors.
An aspect of the present invention resides in a motor control system which comprises brushless motor, a phase detector and a control unit, the brushless motor having multi-phase coils and generating a torque for assisting the operation of an input member, the phase detector detecting an actual phase angle of a rotor of the brushless motor at predetermined angle intervals, and the control unit being connected to the brushless motor and the phase detector, the control unit being arranged to calculate an estimated phase angle on the basis of the actual phase angle and an elapsed time from the detection moment of the actual phase angle detected by the phase detector, to control a command current indicative of a current to be supplied to the brushless motor on the basis of the estimated phase angle and the actual phase angle, and to execute at least one of an estimated phase angle returning processing for returning the estimated phase angle and a current lowering processing for lowering the command current, when the estimated phase angle advances relative to the actual phase angle.
Another aspect of the present invention resides in a method of controlling a brushless motor having multi-phase coils, the brushless motor generating a torque for assisting the operation of an input member, the method comprising the steps of: detecting an actual phase angle of a rotor of the brushless motor at predetermined angle intervals; calculating an estimated phase angle based on the detected actual phase angle detected by the phase detector and an elapsed time from the detection moment of the phase detector; controlling a command current indicative of a current to be supplied to the brushless motor on the basis of the estimated phase angle and the actual phase angle; detecting an advance of the estimated phase angle relative to the actual phase angle; and executing at least one of an estimated phase angle returning processing for returning the estimated phase angle and a current lowering processing for lowering the command current, when the estimated phase angle advances relative to the actual phase angle.
The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.